Thinking about this more this is going to be hard to implement even for the current account. I'll give an example. Let's say you were sorting by top. Let's say you had 2 accounts signed in A on a.com instance and B on b.com instance.

Let's say A is the current account.

You refresh. The app pulls a.com's posts and b.com's posts. Due to b.com being a smaller instance a post (let's call it P) is fetched from b.com on page 1. This post is now shown on the combined view.

You scroll down and now more posts are fetched. Now post P is returned by a.com but on page 2. The issue is, originally P was shown but it was retrieved from account B so you can't comment on it with account A. But now we also have P from account A. By dedupping rules this new post P would not be shown.

There are way around this. Specifically every instance can translate links from other instances to its own instance, however the app will need to make these calls for every post which can be very expensive and taxing on the servers.

There is also the argument to be made that P from b.com should never be shown if P exists on a.com because A is the current user. This essentially forces the app to use the translation API which again is expensive.